The present invention relates to a programmable read only memory device (called hereinafter as a "PROM"), and more particularly to a high speed bipolar PROM.
A memory cell in a bipolar PROM is composed of a transistor of a base open type or a fuse. On a data write operation, the selected memory cell is supplied with a programming current having a current value sufficient to break a PN junction of the base open transistor or to blow the fuse, so that both ends of the memory cell are subjected to be a short-circuit state or an open state.
In the bipolar PROM according to prior art, one end of the memory cell is connected to a bit line and its other end is connected to a word line. Therefore, the programming current having a relatively large value flows from the bit line into the word line via the selected memory cell in order to write a data therein. The programming current should not be supplied to the unselected memory cells. For this purpose, the word lines connected to the unselected memory cells are brought into a current blocking state. Accordingly, a word line driver coupled to the word lines is required, which brings the selected word line into a current drawing state and the remaining word lines into the current blocking state in response to address decode signals from an address decoder.
In a data read operation, a predetermined word line is selected in response to address signals, so that the output data are read out from the memory cells designated by the selected word line.
However, the word line driver is present between the address decoder and the word lines in the prior art. For this reason, the delay caused by the word line driver occurs inevitably from a time point when the application of address signals is initial to a time point when one of the word lines is activated. As a result, a high speed in the data read operation is restricted. In recent years, construction of a bipolar PROM in a gate-array device has been tried, but this has not achieved practical use because of the absence of the bipolar PROM attaining a data read operation at a high speed.